The present product relates generally to cementitious compositions which are particularly useful as wallboard joint compounds, such as are used in finishing gypsum wallboard panels. More specifically, a joint compound is provided in a single dose self contained package having a long shelf life.
Joint compounds are applied to complete the installation of gypsum panels by filling the space between adjacent panels, typically in combination with paper or fiberglass tape. The joint compound is forced into the space between the panels, either before or after the tape is applied. After the space between the panels has been filled, an additional coat or coats of joint compound, either the same as the first or formulated for the purpose, are generally applied to provide a smooth transition between the panels, after which the panels will usually be painted or receive some other surface finish. Joint compounds are also used to repair wall surfaces for the purposes of providing an esthetically pleasing uniform finish ready for final decoration.
Generally, joint compounds include a filler, a binder, and a rheology modifier, typically a thickener. Conventional fillers include calcium carbonate, calcium sulfate dihydrate (gypsum), and calcium sulfate hemihydrate (plaster of Paris). Calcium sulfate hemihydrate sets by hydration to gypsum, while compounds using other fillers harden by simple drying. Thus, joint compounds are usually divided into two types, setting or drying, depending on whether the filler contains a substantial amount of calcium sulfate hemihydrate.
When a powdered setting-type joint compound is mixed with water, the calcium sulfate hemihydrate will react with the water to form gypsum. Typical set times for setting type joint compounds usually are between 5 and 300 minutes and are set by the user in selecting a bag of dry compound having a factory determined setting time. Consequently, water is not typically added to powdered setting-type joint compounds until just prior to use, at which time the setting process begins shortly after the addition of water. Thus, a drawback of setting type joint compounds when used in patching applications is that there is a likelihood of waste, where a portion of the compound often sets before it can be used, or the user mixes too much or too little compound for the particular job.
In addition, there are drawbacks related to the extra step required in mixing a powder with water prior to use. First it requires that potable water be available at the location which is not always the case for new construction. Second, setting type joint compounds can be difficult to mix properly.
To overcome problems of adding water at the time of use, joint compounds have been formulated to prevent the reaction of the hemihydrate with water so that premixed setting-type joint compounds can be manufactured. Setting compounds also have an advantage relative to drying-type joint compounds, since drying-type compounds will shrink during drying, requiring multiple coats and sufficient drying time between coats, making it difficult to achieve a smooth surface suitable for decorating in a timely manner.
Premixed setting-type joint compounds have been suggested in previous patents. For example, U.S. Pat. No. 4,661,161 discloses a set retarding composition in which a proteinaceous ingredient formed from animal tissues and a set preventing agent for calcium ions is used. A set initiator including a compound having a higher log K value than calcium is mixed with the joint compound when setting is desired.
Other ready-mixed, setting type joint compounds are described in U.S. Pat. No. 6,805,741 and in US Patent Publication No. 2008/0229981, which are incorporated by reference.
As noted in U.S. Pat. No. 5,746,822, which is also incorporated by reference, the shelf life of such ready-mixed setting joint compound is not sufficient, since the activity of the set preventer was found to be degraded with time. Thus, the '822 patent discloses an improved setting-type joint compound which includes non-calcium bearing phosphate additives to prevent setting of a premixed calcium sulfate hemihydrate based joint compound which includes water. The shelf life is said to be at least several months.
Thus, there is a need for providing a joint compound with the performance advantages of setting-type compounds without the potential for waste during application due to chemical setting, inconvenience of requiring potable water at the site, and difficulty related to the blending of powders with water via hand mixing.